The present invention relates to high temperature gas-cooled reactors having a core composed of graphite blocks containing nuclear fuel. Such a core serves to moderate neutrons in the chain reaction, to conduct heat from the fuel to a coolant, and to direct coolant from an inlet to an outlet of the reactor core.
A core of this type can be built up from a plurality of individual graphite blocks provided with coolant flow channels. The core dimensions of interest for commercial applications are relatively large, on the order of 3.3 meters in diameter to 3.3 to 6.6 meters in height. To construct such a core, a large number of graphite blocks, each of which is small enough to facilitate the removal and replacement of individual blocks, are assembled together. Typically, several blocks are stacked on top of one another and a plurality of such stacks are disposed adjacent one another to form the core. The blocks are provided with vertical flow passages which are placed in alignment to form continuous coolant flow channels which extend from the bottom to the top of the core.
Graphite is a relatively good thermal conductor and has a high melting point, of the order of 3500.degree. C. When coupled with a high melting point fuel, which may be in the form of pellets embedded in the graphite, such a reactor appears to be relatively immune to accidents which could melt the core.
However, the ability of the core to resist melting depends on the maintenance of a significant flow of coolant, typically helium, through the core during power operations.
While the provision of a substantial number of vertical coolant channels offers a reasonably reliable cooling action, a blockage occurring in any one vertical channel can lead to local heating and blockage of a sufficient number of channels could initiate core melting. Such a blockage could be produced, for example, by foreign material accidentally dropped into the core and/or accumulating below the core, the breaking off of parts within the core enclosure, the breaking off of a piece from a graphite block, misalignment of graphite blocks, or the accumulation, within a channel, of debris or impurities suspended in the coolant fluid.
If blockage should occur in a channel, localized heating can occur not only at the blockage, but in the core region adjacent the channel and upstream and downstream of the blockage.